The purpose of the proposed research is to examine individual differences in infants by studying a behavioral response (visual recognition memory) and physiological activity (heart rate variability). Visual recognition memory and heart rate variability (i.e., vagal tone) have each been shown to differentiate between groups of infants of different ability levels and to correlate with later cognitive functioning. In the proposed study, both of these measures will be used to determine individual differences in a group of normal infants. The relationship between visual recognition memory and heart rate variability will also be examined. In addition, the stability of both visual recognition memory and heart rate variability (i.e., vagal tone) will be investigated. The Bayley Scales of Infant Development will be used as developmental markers. Infants (fullterm, Caucasians) will initially be tested when they are 24 to 48 hours old. Heart rate and respiration will be measured for 10 minutes. At 3 and 6 months, both visual recognition memory and physiological activity will be assessed. A baseline recording of heart rate and respiration will be made for 10 minutes. Visual recognition memory will be assessed while heart rate and respiration are recorded. The paired comparison paradigm of visual recognition memory with fixed study times will be utilized. The Bayley Scales of Infant Development will be administered within 5 days of the visual recognition memory testing. Visual recognition memory will be quantified as a percentage of the novelty response, duration of the familiarization period, total looking time at novel stimuli, total looking time at familiar stimuli, and time not looking during the test trials. Heart rate variability will be analyzed with spectral analysis to derive a measure of vagal tone (V). Other variables of heart rate will be examined (i.e., mean heart period, short-term heart rate variability, and long-term heart rate variability).